Orthopedic implants are used for a variety of joint replacements and to promote bone repair in humans and animals. According to medical industry analysts, there are now over 800,000 hip and knee joint replacements performed in human patients each year in the U.S. In addition, hundreds of thousands of human patients undergo surgical procedures in which orthopedic implants are used, for example, to treat various types of bone fractures or to relieve severe back pain.
With all of these procedures, there is a need for controlled, directed, rapid healing. Individuals undergoing joint replacement often experience uncomplicated healing and restoration of function. Unfortunately, there is a high rate of complications, including “late failures.” The revision surgery rate for human total joint replacement varies between 10 to 20% (Malchau et al. (2002) “Prognosis of total hip replacement: Update of results and risk-ratio analysis for revision and re-revision from the Swedish National Hip Arthroplasty Registry, 1979-2000,” scientific exhibition at the 69th Annual Meeting of the American Academy of Orthopaedic Surgeons, Dallas, Tex., Feb. 13-17, 2002; Fitzpatrick et al. (1998) Health Technol. Assess. 2:1-64; Mahomed et al. (2003) J. Bone Joint Surg. Am. 85-A:27-32)). The majority of these revision surgeries are made necessary by failure at the implant-bone interface.
Orthopedic implants are made of materials which are relatively inert (“alloplastic” materials), typically metallic, ceramic, or plastic materials. Previous approaches to improve the outcomes of orthopedic implant surgeries have mainly focused on physical changes to the implant surface that result in increased bone formation. These approaches include using implants with porous metallic surfaces to promote bone ingrowth and spraying implants with hydroxyapatite plasma. Approaches using dental implants have also included the use of topographically-enhanced titanium surfaces in which surface roughness is imparted by a method such as grit blasting, acid etching, or oxidation. While these techniques have improved the outcomes of orthopedic implant surgeries, there is still considerable room for further improvement.
Tissue response to an alloplastic material is known to be influenced by cell adhesion to the material's surface, and much research has been directed to improving cell adhesion to alloplastic materials. Cell adhesion between cells in vivo is known to be controlled primarily by the binding of short, exposed protein domains in the extracellular matrix to cell surface receptors (LeBaron & Athanasiou (2000) Tissue Eng. 6: 85-103; Yamada (1997) Matrix Biol. 16: 137-141). Notably, a class of receptors known as integrins has been implicated in cell adhesion to implant surfaces. Integrins and their target ligands have been shown to stimulate osteoblast adhesion and proliferation as well as bone formation (see, e.g., Kantlehner et al. (2000) ChemBioChem 1: 107-114; Sarmento et al. (2004) J. Biomed. Mater. Res. 69A: 351-358; Hayashibara et al. (2004) J. Bone Mineral Res. 19: 455-462. Integrins may be useful in targeting cell adhesion to implants and in this manner may improve integration of implants into adjacent bone.
Other research has shown that the local expression of growth factors and cytokines can enhance tissue reactions at alloplastic implant surfaces. For example, Cole et. al. ((1997) Clin. Orthop. 345: 219-228) have shown that growth factors can promote the integration of an implant into adjacent bone (“osteointegration”) as well as increase the rate of bone formation next to the implant surface. See also U.S. Pat. No. 5,344,654. Growth factors that stimulate new bone production (“osteoinductive proteins”) include, but are not limited to, platelet-derived growth factor (PDGF), insulin-like growth factors 1 and 2 (IGF-1 and IGF-2), vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), transforming growth factor (TGF-β), bone morphogenic proteins (BMP), and associated family members.
The most effective osteoinductive proteins are the bone morphogenetic proteins (BMPs). The BMPs are members of the TGF-β superfamily that share a set of conserved cysteine residues and a high level of sequence identity overall. Over 15 different BMPs have been identified, and most BMPs stimulate the cascade of events that lead to new bone formation (see U.S. Pat. Nos. 5,013,649; 5,635,373; 5,652,118; and 5,714,589; also reviewed by Reddi and Cunningham (1993) J. Bone Miner. Res. 8 Supp. 2: S499-S502; Issack and DiCesare (2003) Am. J. Orthop. 32: 429-436; and Sykaras & Opperman (2003) J. Oral Sci. 45: 57-73). This cascade of events that leads to new bone formation includes the migration of mesenchymal stem cells, the deposition of osteoconductive matrix, the proliferation of osteoprogenitor cells, and the differentiation of progenitor cells into bone-producing cells. Much research has been directed to the use of BMPs on or near implants in order to promote osteointegration of the implants (see, e.g.: Friedlander et al. (2001) J. Bone Joint Surg. Am. 83-A Suppl. 1 (Pt. 2): S151-58; Einhorn (2003) J. Bone Joint Surg. Am. 85-A Suppl. 3: 82-88; Burkus et al. (2002) J. Spinal Disord. Tech. 15(5): 337-49). However, one of the critical issues that remains unresolved is the method of grafting or immobilizing an active BMP or other active biomolecule onto the surface of an implant.
It has been shown that the presentation of BMPs is critical for producing desired bone formation next to an implant device. Approaches to improving implants have been modeled in view of the natural process of bone formation. In human bone, collagen serves both as a scaffold for bone formation and as a natural carrier for BMPs. Demineralized bone has been used successfully as a bone graft material; the main components of demineralized bone are collagen and BMPs (see U.S. Pat. No. 5,236,456). Many matrix systems have been developed that are designed to encourage bone formation by steadily releasing growth factors and other bioactive molecules as the matrix degrades. The efficiency of BMP release from polymer matrixes depends on matrix characteristics such as the affinity of BMP for the matrix, resorbtion rate, density, and pore size. Materials used in such matrix systems include organic polymers which readily hydrolyze in the body into inert monomers. Such organic polymers include polylactides, polyglycolides, polyanhydrides, and polyorthoesters (see U.S. Pat. Nos. 4,563,489; 5,629,009; and 4,526,909). Other materials described as being useful in BMP-containing matrices include polylactic and polyglycolic acid copolymers, alginate, poly(ethylene glycol), polyoxyethylene oxide, carboxyvinyl polymer, and poly(vinyl alcohol) (see U.S. Pat. No. 5,597,897). Natural matrix proteins have also been used to deliver BMPs to bone areas; these natural proteins include collagen, glycosaminoglycans, and hyaluronic acid, which are enzymatically digested in the body (see U.S. Pat. Nos. 4,394,320; 4,472,840; 5,366,509; 5,606,019; 5,645,591; and 5,683,459).
Even with the use of a polymer matrix to retain BMP at the site of repair, it has been found that supraphysiological levels of BMP are required in order to promote healing due to the rapid diffusion of growth factors out of the matrix. For example, with a collagen sponge delivery system, only 50% of the BMP added to the sponge is retained after two days (Geiger et al. (2003) Adv. Drug Del. Rev. 55: 1613-1629). The high initial dose of BMPs required to maintain physiological levels of BMP for the necessary period of time makes BMP treatment more expensive and may lead to detrimental side effects such as ectopic bone formation or allergic reactions, or the formation of neutralizing antibodies.
Similar problems exist with other implants such as tendon and ligament replacements, skin replacements, vascular prostheses, heart pacemakers, artificial heart valves, breast implants, penile implants, stents, catheters, shunts, nerve growth guides, intraocular lenses, wound dressings, and tissue sealants. As with orthopedic implants, surgery involving these implants often gives rise to similar problems with the slow healing of wounds and, where desirable, improper integration of the implant into surrounding tissue.
Thus, there remains a need for the development of cost-effective methods for grafting active biomolecules to the surface of materials used as implants or in conjunction with implants in order to promote post-surgical healing and, where desirable, integration of the implant into surrounding tissues, such as, for example, adjacent bone.